Water harvesting and soil moisture retention - chapter 7,8 pptx

Water harvesting and soil moisture retention
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Part II: Soil moisture retention
In this second part soil moisture retention techniques to be applied in
the cultivated area are described. A distinction is also made here be-
tween systems which follow the contour lines of a slope and those
which are independent of contour lines. Chapter 7 deals with contour
systems to improve infiltration. Chapters 8 and 9 describe water con-
servation measures, which are not necessarily contour-bound.
7 Contour systems to improve
infiltration
Contour farming is a term used to include ploughing, furrowing and
planting along the contours of a hill side. The objective of contour
farming is to increase infiltration into the soil along the contours and
to conserve soil moisture there. Contour farming may reduce runoff
and soil erosion by as much as 50%.
The first step in contour farming is to determine a contour guide line.
One method for marking out contour lines, the water tube-level, is de-
scribed in Appendix 3. Several other techniques are described in
Agrodok No.6 'Field surveying'.
All subsequent water conservation measures are related to the contour
guide lines. Hedges, shrubs or stones may be used to mark these lines.
In small fields or on even slopes one guide line may be sufficient. This
line should lie about halfway up the slope. On irregular slopes, or in
large fields, more guide lines are necessary. In this case the various
contour lines should be evenly distributed over the slope.
7.1 Contour ploughing
Contour ploughing ensures that rainfall and runoff water are spread
evenly over a field by making furrows parallel to the contours.

Contour systems to improve infiltration
63
Conditions
Contour ploughing may be done on slopes with a gradient of less than
10%. On steeper slopes it is better to combine contour ploughing with
other measures such as terracing or strip cropping.
Contour ploughing is practical on fields with an even slope. On very
irregular slopes it is too time-consuming to follow the contours when
ploughing. Strip cropping (see next paragraph) is then often more ef-
fective.
Contour ploughing can be risky when the soil takes up water only
slowly (e.g. soils with a high clay content, with impermeable layers or
shallow soils). Furrows should not be longer than 100 m and, if they
are graded, the slope should be less than 1%.
Procedure
After laying out a contour line, plough
the first row along this line. On an ir-
regular slope and other slopes where
several guide lines are laid out, plough-
ing follows the pattern shown in Figure
27.
? Plough parallel to each contour guide
line, always taking the nearest con-
tour guide line as a reference point.
? Plough shorter rows each time, leav-
ing a rectangular strip in the middle
to turn around. The most suitable
number of long rows is 4 to 6 on
steep ground, 7 to 10 on more grad-
ual slopes.

? Finally, plough the space used for
turning, in straight lines.
Existing gullies on slopes are better left
unploughed, because soil erosion might
be encouraged otherwise. It is often

Figure 27: How to plough
on a field with several con-
tour guide lines.

Water harvesting and soil moisture retention
64
necessary to construct a spillway to enable excess water to be shed
safely.
Furrows may be laid out at a slight angle, e.g. a gradient of 1%, so that
runoff water can be collected in a discharge drain. If slopes are less
than 15%, simple grassed channels are sufficient, but on steeper slopes
more sophisticated structures are required, e.g. a lock-and-spill drain
(see Appendix 1).
7.2 Strip cropping
Strip cropping means cultivating different types of crops in strips fol-
lowing the contours (Figure 28). Generally a good ground cover crop
is alternated with a crop that provides little ground cover. The ground
cover strip slows down the flow of rainwater down the slope and pre-
vents it from washing away valuable topsoil. The water can then be
used by the (exposed) crops in the next strip. By tilling only the strips
that are to be planted, the farmer saves labour. Strip cropping differs
from vegetation strips in that vegetation strips are narrower and per-
manent, while crops are often rotated in strip cropping.
Figure 28: Strip cropping.

Contour systems to improve infiltration
65
Conditions
Strip cropping is usually applied to slopes not steep enough to warrant
terracing. On its own it can be carried out on slopes with a gradient of
up to 5%. If the gradient of the slope is steeper, strip cropping should
be combined with other measures such as (tied-)ridging and mulching.
On soils where infiltration is difficult (clay soils and soils with a
crust), it is better to combine strip cropping with ridging.
Selection of crops
The success of strip cropping depends on careful choice of crops. As
far as possible they should not compete with each other for water and
nutrients. It is useful to combine crops that differ in their ground cov-
erage, and which have different growth cycles. In this way their peak
water requirements and harvest periods will come at different times.
Combinations of grasses and legumes are common, as well as of cere-
als and creeping legumes, e.g. millet and groundnuts (Figure 28). An
advantage of many legumes is that they fix nitrogen, which may im-
prove overall fertility of the soil.
Layout
The width of the strips depends on the slope gradient and the infiltra-
tion capacity of the soil. Table 9 below gives guidelines for the width
of strips on reasonably permeable soils (i.e. soils which are not

clayey).
Table 9: Strip cropping: relation between width and slope.
Slope gradient Width of strip
0-2% 40-50 m
2-4% 30-40 m

> 4% 15-30 m
in very humid climates 10-30 m
Where machinery is used, the width of the strips depends on the size
of the machinery. The width of the strip will be an exact multiplication
of the width of the machines. If slopes are irregular, the strips with
arable crops are kept the same width, while the irregularities of the

Water harvesting and soil moisture retention
66
slope are corrected in the "buffer" strips (the strips with grasses, cover
crops, etc.).
Maintenance
Grass strips have to be cut back periodically. The strips with arable
crops and those with grasses or cover crops can be rotated to maintain
fertility of the soil and to combat pests and weeds.
7.3 Ridging and tied-ridging
Ridging is done by constructing small earth banks parallel to the con-
tours of a slope. The water accumulates above the ridges and is thus
allowed to infiltrate into the soil. An alternative to ridges is the con-
struction of small earth mounds.
Conditions
This method of soil moisture conservation is used on slopes with a
gradient of up to 7%. Soils should have a relatively stable structure,
otherwise the ridges become undermined by runoff and will be de-
stroyed. Ridging requires more labour and capital investment than
strip cropping.
Size and shape
The height of the ridges is usually 20-30 cm. The ridges are as wide as
the furrows. The distance between ridges varies from 1.5 to 10 m, and
depends on the crop grown, the steepness of the slope and the climate.

The distance between ridges can be larger, if combined with strip
cropping.
In areas with heavy rainfall, there is a risk of crops becoming water-
logged or the ridges being washed away. This can be prevented by
making the ridges sloping a little downwards, at a slight angle to the
contour line. In this way the water can be diverted into a drainage
channel.
Tied-ridging
A variation on ridging is the partitioned furrow technique, better
known as tied-ridging. In this system lower ridges, cross-ties

Contour systems to improve infiltration
67
(15-20 cm high), are made every few metres across the contour fur-
rows, creating mini-basins (Figure 29). In case of light rainfall, the
water remains in the mini-basins. When rainfall is heavy, the water
runs off over the cross-ties along the contour, because the cross-ties
are lower than the ridges and the furrows are built at an angle to the
contour. Thus overtopping, i.e. excess water flowing over
the ridges, is
prevented. The cross-ties reduce the speed of the water flow.
Figure 29: Tied-ridging with ridges built at a slight angle to the con-
tour line.
Conditions
Tied-ridging can be used only where rainfall does not exceed the stor-
age capacity of the furrows; otherwise severe erosion may be the re-
sult. Tied-ridging is more successful on coarser soils (more sandy),
which are less prone to waterlogging, for example on alfisols in the
Sudano-Sahelian tropics. Vertisols, black soils with a high clay con-
tent, give better overall production yields where broad-bed and furrow

techniques are used (see next paragraph).
Planting configuration
Seeds or tubers are placed either near the top of the ridge (to avoid
water logging) or towards the bottom of the basin where rainfall
and/or soil moisture are limited. The most appropriate site for planting
also depends on the water requirements of the crop.

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Maintenance
The construction and maintenance of ridges is hard work, especially
on heavy (clayey) soil. In order to spread the work out, in the first year
the contour ridges can be ploughed using an ox-plough or tractor-
drawn implement with a reversible blade, and the cross-ties can be
made by hand. Ploughing and ridge making only have to be repeated
once every four or five years. This makes total labour input suffi-
ciently low.
7.4 Broad-bed and furrow
The purpose of a broad-bed and furrow system is to increase the
amount of water that infiltrates into the soil and that is stored in both
bed and furrow. It also makes heavy soils more workable by improv-
ing drainage and extending the time of infiltration. When rainfall is
very heavy, the (grassed) furrows carry runoff water away, because
they slope down with a slight gradient. Another advantage of a broad-
bed and furrow system is that it makes mixed cropping or intercrop-
ping possible.
Conditions
This system is mostly used in areas with intense rainfall (a yearly av-
erage of 750 mm or more) and on black clay soils (vertisols), where
water infiltration is very low. These soils are deep and have a large

water storage capacity. Gently sloping land (0.5-3%) is most appropri-
ate. The system is not
suitable for red soils (alfisols) or shallower
soils.
Size and shape
The broad-bed and furrow system consists of broad beds of approxi-
mately 100 cm wide, separated by furrows of about 50 cm wide (Fig-
ure 30). Bed width and planting configuration vary according to the
cultivation and planting equipment available. Two to four rows of
crops are usually planted on one bed. Animal-drawn equipment (a bul-
lock-drawn moulder) can be used to make the beds.

Contour systems to improve infiltration
69
Figure 30 shows a narrow bed and furrow and two variations of a
broad-bed and furrow. It is clear that the broad-bed and furrow system
makes it possible to combine different crops and planting densities.
Planting is carried out in 2, 3 or 4 rows with 75 cm, 45 cm or 30 cm
row spacing, respectively. Figure 30A shows a maize crop in a narrow
ridge and furrow system (planting distance 75 cm). Figure 30B gives
an example of a broad-bed and furrow using the same crop and plant-
ing distance. Figure 30C shows a combination of maize and pigeon
pea, with a planting distance of 45 cm.
The furrows are often planted with grasses to avoid soil erosion and
they slope down at an angle of between 0.4% and 0.8% along their
length, depending on the gradient of the slope.
Figure 30: Broad-bed and furrow system using combinations of
different crops and different planting densities.

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70
8 Measures to improve infiltration
and water storage
Infiltration of water into a soil is improved by making the soil struc-
ture looser and the top layer more rough. This can be achieved through
the use of cover crops or mulching and tillage. These three measures
are described here.
8.1 Cover crops
Cover crops are usually creeping legumes which cover the ground sur-
face between a widely spaced perennial crop, such as young fruit
trees, coffee, cacao and oil palms. Cover crops are often combined
with mulching. Grass is also used as ground cover between orchard
terraces, i.e. narrow terraces for fruit trees, with intermittent unculti-
vated strips (Figure 31).
Figure 31: The use of cover crops on terraces.
Cover crops protect the soil from splashing raindrops and too much
heat from the sun. They build up organic matter in the soil, they im-
prove soil structure and they may increase soil fertility through nitro-
gen fixation. Cover crops also suppress weed growth.

Measures to improve infiltration and water storage
71
Conditions
Cover crops are not very suitable for areas where average annual rain-
fall is less than 500 mm because competition with the main crop for
water might occur. In these areas it may be better to let the weeds
stand, provided they do not overrun the main crop.
Legumes are fairly susceptible to disease and often need to be fertil-
ized with phosphorus.

Layout
Cover crops may either cover the whole area between fruit trees
(overall covering), or they may be grown in strips (strip covering) be-
tween the tree rows. Strip covering is better for young trees. Figure 32
provides examples of both overall covering and strip covering by
cover crops combined with mulching.
Figure 32: A combination of cover crops and mulching.
Criteria for selecting a cover crop:
1 Easily propagated by seed.
2 Grows rapidly without competing with the main crop.
3 Tolerates some shade and cutting back around the crop.
4 Does not act as a host to pests and diseases attacking the main crop.
This risk can be limited by choosing crops from different families.
5 Suppresses weed growth.

Water harvesting and soil moisture retention
72
6 Has additional productive functions as food (e.g. groundnuts,
beans), animal feed, mulch, etc.
Some cover crops are drought resistant, e.g. Centrosema pubescens,
Pueraria phaseoloides, Stylosanthes gracilis. Some legumes are also
effective pesticides (e.g. Tephrosia candida).
8.2 Mulching
Mulching is done by covering the soil between crop rows or around
trees with grass, straw, crop residues or other plant material. When
crop residues are left on and in the soil after harvesting, this is called
stubble mulching.
The mulch layer is rougher than the surface of the soil and thus inhib-
its runoff. The layer of plant material protects the soil from splash ero-
sion and prevents the formation of a crust. Where soil is covered by a

mulch layer, evaporation is reduced, because the upward movement of
soil moisture is hampered. Another effect of a mulch layer is that soil
temperature is kept constant, which means that more micro-organisms
survive the dry period. Finally, weed growth is also kept down by
mulching. Surface rooting crops benefit particularly from mulching, as
their roots are found in the partly decomposed layer between soil and
mulch.
Conditions
The soils should have good drainage. Areas with marginal rainfall
usually respond better to mulching with dead
organic material than to
cover crops, because mulch does not compete for water and nutrients.
Layout
Mulch can be spread on a seedbed or around planting holes. This is a
good practice for trees and crops which require watering. In Senegal
mulching of planting holes reduced the watering requirements of to-
matoes from once a day to once every three days.
Mulch can also be applied in strips (Figure 32). Alternative row
mulching is sometimes preferred to full mulching, because it reduces

Measures to improve infiltration and water storage
73
the fire risk. The layer should not be too thick; otherwise the soil un-
derneath heats up.
Use a mixture of fast and slow decomposing material. Break or cut
large pieces of crop residue before application. Cover crops or grasses
in orchards are a readily available source of mulch material. Grass for
mulching should be allowed to dry before applying as this not only
reduces the weight to be carried, but also the chance of the grass root-
ing. The mulch may be covered with a layer of soil to protect it against

wind. During sowing or planting the mulch is lifted to one side and the
planting hole is covered afterwards.
A variation of this technique, used in combination with micro-
catchments or tied-ridging, is vertical mulching. Straw or stubble are
buried in a narrow trench in the topsoil, at the spot of water concentra-
tion and in contact with the air (Figure 33). This enables water to be
rapidly channelled into the soil by providing slots which are open to
the air. Tillage should be avoided.
Figure 33: Vertical mulching
Constraints
? Mulching requires a large amount of plant remains, not always
available to small-scale farmers in dry areas. Mulch is most effec-
tive if applied at the start of the rains, as it intercepts and increases

Water harvesting and soil moisture retention
74
water take-up, but it is frequently more practical to mulch towards
the end of the rains when grass is available. In addition, organic
mulches decompose rapidly in high temperature climates.
? The constant soil temperature and humidity may enable diseases
and pests to survive from one rainy season to the next.
? Mulching of dried grasses may be a fire hazard. This can be reduced
by working the mulch into the soil, by alternative row mulching or
by making fire pathways around the field.
? In some countries it is customary to burn crop remains just before
the rainy season starts. This releases large amounts of nutrients that
are available at once for the next crop. When crop remains are used
for mulching, these nutrients are released more slowly, so that more
manure or fertilizer has to be applied.
8.3 Tillage

There is much discussion about the effect of tillage on soil moisture
conservation. Tillage is good for water infiltration and root penetra-
tion, as the soil is worked into clods. However, this is only true for
stable soils. If the soil is less stable, the clods will disappear rapidly
when it rains.
Tillage is required on badly degraded soils or for those that undergo
severe hardening during the dry season. Deep tillage (disturbing the
soil below 10 cm) has proven beneficial on dense sandy soils in Bot-
swana.
However, repeated cultivation to the same depth may cause a com-
pacted soil layer to form at the bottom of the tilled layer (called a
'plough-pan', or 'hoe-pan' etc.). Plant roots cannot penetrate into this
layer and the water storage capacity of the soil is reduced.
In this case, when the clogged layer is several tens of centimetres be-
low the surface, subsoiling is necessary to increase infiltration (Figure
34B).
Some soils become crusted over on the surface when it rains, espe-
cially soils containing much clay and silt. This leads to a low infiltra-
tion rate and a high rate of runoff.

Measures to improve infiltration and water storage
75
In this case, with crusted soils, when the soil pores are clogged in the
first few millimetres or centimetres, hoeing or superficial ploughing is
sufficient to break up the crust and let the water infiltrate (Figure
34A).
Figure 34: Breaking up clogged soil layers.
Constraints of tillage:
? It can encourage soil erosion and more rapid decay of soil organic
matter.

? It may allow more moisture to escape through evaporation.

Water harvesting and soil moisture retention
76
8.4 Minimum-tillage and zero-tillage
In some situations it may be better to confine soil tillage to a minimum
(minimum-tillage) by leaving the stubble after harvesting and only
ploughing just before the next crop is planted or sown. It is also possi-
ble not to plough at all, but just to make holes to plant the following
crop (zero-tillage). Both limit runoff and prevent loose soil material
from forming a crust. Moreover they are labour-saving, increase soil
organic matter and prevent erosion.
Conditions
Soils should not be susceptible to compaction or crusting. They should
be well draining (i.e. not too clayey), have high biological activity, a
crumbly consistency and a coarse surface.
Constraints
? The existing vegetation may compete with the crops for water and
nutrients.
? These systems often lead to a weed problem.
? Insects may thrive in plant residues.